1. Field of the Invention
The present invention relates to a writable optical disk, and more particularly to the technology for optical disks having meandering guide grooves, which enables proper reproduction of signals from the meandering guide grooves without influence of adjacent tracks.
2. Related Background Art
For example, a writable compact disk has guide grooves recorded thereon, which meander at a constant carrier frequency of 22050 Hz. When the meandering guide grooves are reproduced, a reproduced signal is obtained with a certain frequency because of fluctuation in light quantity of reflected light. Thus, a linear velocity of the disk can be controlled at a constant value by adjusting a rotational velocity of the disk so as to maintain constant the frequency of reproduced signal corresponding to the fluctuation in the quantity of reflected light. Since the meandering guide grooves include time information, high-speed search of arbitrary position is possible on the disk, using the time information measured from the meandering guide grooves for example.
FIG. 4 and FIG. 5 show the meandering guide grooves 1 formed on a disk. FIG. 4 shows a state in which adjacent guide grooves meander in phase with each other, that is, the appearance of meandering guide grooves formed in an in-phase state. FIG. 5 shows a state in which adjacent guide grooves meander in antiphase to each other, that is, the appearance of meandering guide grooves in an out-of-phase state. Such a conventional optical disk has a track pitch of p=1.6 .mu.m and a meander amplitude of a=0.03 .mu.m, so that a ratio therebetween p/a is approximately 53 and is maintained constant thereat.
Incidentally, the in-phase state and the out-of-phase state as described above alternately appear at a frequency f.sub.beat (HZ). The frequency f.sub.beat may be approximated by the following equation, if the carrier frequency of meander is f.sub.wob (Hz), the linear velocity in writing or reproducing data is v.sub.CLV (m/s), the track pitch of disk is P (.mu.m) and a radial position on disk is r(mm). EQU f.sub.beat =(1/2)[(1/(2f.sub.wob)).multidot.(1/P).multidot.(10.sup.3).multidot.55 r+(1/(8f.sub.wob)).multidot.(v.sub.CLV /.pi.).multidot.(10.sup.3)}-1/(4f.sub.wob)].sup.-1 (Hz)
Since for current writable compact disks f.sub.wob =22050 (HZ), V.sub.CLV =1.2 (m/s), P=1.6 (.mu.m) and r=25-58 (mm), the frequency f.sub.beat is approximately 1.4 (Hz) to 0.6 (Hz).
Meanwhile, it is the case in such a writable compact disk that an amplitude of the reproduced signal on reproducing guide grooves in the in-phase state as shown in FIG. 4 is different from that on reproducing guide grooves in the out-of-phase state as shown in FIG. 5. Specifically, FIG. 6 shows a reproduced signal on reproducing guide grooves in the in-phase state, while FIG. 7 shows a reproduced signal on reproducing guide grooves in the out-of-phase state. Namely, leakage of signals from adjacent guide grooves causes such influence that the amplitude of reproduced signal on reproducing the guide grooves in the in-phase state is greater than that on reproducing the guide grooves in the out-of-phase state. Therefore, when such grooves are continuously reproduced, a reproducing signal is output while changing its amplitude depending upon the phase condition, i.e., upon the in-phase state or the out-of-phase state, as shown in FIG. 8.
If there occurs a defocus of the reproducing pickup, unevenness of width or depth of the guide grooves, unevenness of track pitch, unevenness of reflectivity of the recording film, or unevenness of thickness of the disk substrate, the influence is further enhanced to cause problems such as an error in reading a signal of the meandering guide grooves, failure in writing or reproducing correct information, and inability of high-speed search.